Wednesday, February 24, 2010

Early Vision More Complicated

As you read these words a frenzy of activity is taking place as the light entering your eye triggers a dizzying sequence of actions, ultimately causing a signal to be sent to your brain. In fact, even a mere single photon can be detected in your vision system. It all starts with a photon interacting with a light-sensitive chromophore molecule. The interaction causes the chromophore to change configuration and this, in turn, influences the large, trans-membrane rhodopsin protein to which the chromophore is attached. This is just the beginning of the cellular signal transduction cascade.

The chromophore photoisomerization is the beginning of a remarkable cascade that causes action potentials to be triggered in the optic nerve. In response to the chromophore photoisomerization, the rhodopsin causes the activation of hundreds of transducin molecules. These, in turn, cause the activation of cGMP phosphodiesterase (by removing its inhibitory subunit), an enzyme that degrades the cyclic nucleotide, cGMP.

A single photon can result in the activation of hundreds of transducins, leading to the degradation of hundreds of thousands of cGMP molecules. cGMP molecules serve to open non selective ion channels in the membrane, so reduction in cGMP concentration serves to close these channels. This means that millions of sodium ions per second are shut out of the cell, causing a voltage change across the membrane. This hyperpolarization of the cell membrane causes a reduction in the release of neurotransmitter, the chemical that interacts with the nearby nerve cell, in the synaptic region of the cell. This reduction in neurotransmitter release ultimately causes an action potential to arise in the nerve cell.

All this because a single photon entered into the fray. In short order, this light signal is converted into a structural signal, more structural signals, a chemical concentration signal, back to a structural signal, and then back to a chemical concentration signal leading to a voltage signal which then leads back to a chemical concentration signal. There is, of course, a wealth of yet more detail which makes the information conversion process far more complicated.

Cellular signal transduction design is modular. Its many steps can be modified, or interchanged with alternative steps to provide solutions in other applications, such as the olfactory system. Within the vision system one can, for instance, modify the chromophore's color sensitivity—its action spectrum—so different colors cause their own specific signals.

An example of this is found in the so-called third eye (parietal eye) which is found in a variety of species. This eye is not an image forming eye but rather provides for light sensitivity. This system includes two antagonistic light signaling pathways in the same cell. Blue light causes the hyperpolarizing response as described above, but green light causes a depolarizing response.

How is this done? By the inhibition of the cGMP phosphodiesterase enzyme. Specifically, there are two opsins, one that is sensitive to blue light which activates the cGMP phosphodiesterase enzyme, and another that is sensitive to green light which inhibits the cGMP phosphodiesterase enzyme. It appears that initially these are two separate pathways and they come together at the point of influencing the cGMP phosphodiesterase enzyme.

The molecular components of this fascinating design are elucidated in a 2006 paper. In addition to reporting on their findings of this unique design, the final paragraphs propose an evolutionary explanation for the design. Here the paper turns from empirically based science to unfounded, non scientific speculation. Not surprisingly their evolutionary story begins with the heavy-lifting already accomplished and, in Lamarckian fashion, improvements are implemented as needed:

A G_o-mediated phototransduction pathway might already be present in the ciliary photoreceptors of early coelomates, the last common ancestor of lizard (vertebrate) and scallop (mollusk), because both have this pathway. Later, the ancestral vertebrate photoreceptor acquired a second G protein, either gustducin or transducin, for chromatic antagonism and perhaps other purposes. The parietal photoreceptor evolved subsequently and retained these ancestral features.

One can hardly blame evolutionists for their smuggling in of Lamarckian terminology. It sounds better than the Darwinian just-add-water account which holds that random biological variation produced a phototransduction pathway, and then produced myriad new proteins, which fortunately just happened to include a second G protein, which fortunately just happened to ... well you know the story.

@Ritchie: I think the following toward the end of this post makes things quite clear:

"In addition to reporting on their findings of this unique design, the final paragraphs propose an evolutionary explanation for the design. Here the paper turns from empirically based science to unfounded, non scientific speculation. Not surprisingly their evolutionary story begins with the heavy-lifting already accomplished"

I understand Dr. Hunter's point exactly. He described an extremely complex signal transduction system that the scientists accept without question came about by Darwinian processes.

Where is the detailed analysis of what is actually required of a step-by-step evolutionary process to bring about this signal transduction system? I grant that such an analysis would no doubt require a gargantuan effort, but at the same time to accept that all this can actually be accomplished by the putative Darwinian mechansism is naive at best.

Am I making an argument from incredulity? In one sense, yes. But I would rather say that I am questioning the lack of legitimate skepticism that should accompany any evolutionary scenario that is accepted because it has "already been established" that the theory of evolution is true.

We who are skeptics do not accept that the theory of evolution is true. We (I) would like to see some kind of palusibility analysis as a minimum: the number of genetic changes required to effect a given new feature, a realistic assesment of what are the minimum selectable functions for each step toward that new feature (and how many genetic changes that entails), and given the known rates of mutation, the amount of time required for all these changes to take place.

Or more succintly, where is the heavy-lifing that is needed to justify an evolutionary scenario?

Who cares? The flip-side of Dawkins' amusing "argument from incredulity" jibe is that he is actively advocating for credulity (selective, of course, and special-pleading) with respect to Darwinistic "explanations," that is, for a non-critical approach to the Just-So stories of which "Darwinists" are far too prone and far too fond.

All good "Darwinists" are (selectively) credulous -- which seems a rather odd stance for persons who hold themselves to be amongst the intellectual elite.

"Am I making an argument from incredulity? In one sense, yes. But I would rather say that I am questioning the lack of legitimate skepticism that should accompany any evolutionary scenario that is accepted because it has "already been established" that the theory of evolution is true."

Okay, I see that. But at the same time, isn't that just saying "this paper hasn't provided a detailed analysis of what is actually required of a step-by-step evolutionary process to bring about this signal transduction system"?

Because I'm sure there are LOTS of papers in the world which don't contain this specific information.

What would you accept as an alternative wording of the final paragraph?

"The designer inserted a related a G_o-mediated phototransduction pathway in the ciliary photoreceptors of early coelomates, the last common ancestor of lizard (vertebrate) and scallop (mollusk). The designer later inserted a second vertebrate photoreceptor-coupled G protein G protein, either gustducin in some lineages or transducin in others, for chromatic antagonism and perhaps other purposes. The redesigned parietal photoreceptor retained these ancestral features because the designer liked it that way."

Evolution is the best framework. The conservation of ancestral features is suggestive of this.

No, and consequently the unwarranted and unsupported speculation should've been left out.

"Evolution is the best framework." --Depends on what one means by evolution. Certianly neoDarwinian evolution is not the best framework, but most researchers have been too focussed on it to research potential other mechanisms for evolution.

Furthermore, if no mechanistic process for generation of the pathway can be found, then the alternative wording would be a possibility, given that it is logically and rationally more likely than not that one exists. It is no different, in principle, than an agent like me moving an inert spoon, or lab rats cutting up and reforming genetic material.

At the very least ID has forced the concept of information onto the agenda of biological research. Scientists have not yet begun to really plumb the significance of the information that lies within the genetic code.

"No, and consequently the unwarranted and unsupported speculation should've been left out."

But it isn't unwarranted or unsupported. It is a theory which is supported by a vast amount of evidence and it is therefore sensible to assume as a given.

Surely all Cornelius is done here is to show that a particular paper has simply assumed the truth of evolution - which is a point I doubt anyone will argue with. This appears to be true. But that is because it is a reasonable thing to do.

Even if one believes that Darwinian evolution is true, the last paragraph was speculative and did not propose any testable hypothesis for further research. It was pure blue skying.

It is evident from the geological record that biological life did increase in numbers and kinds and complexity over time. It is also evident that we share a great deal of DNA. It is also evident, however, that Darwinian evolution cannot account for this.

I've not heard the term blue skying before. You learn something new every day. :)

As for your assertion, I am at a loss to see why you think Darwinian evolution cannot account for the increases in numbers and complexity of life. It accounts for it very elegantly.

Far more creatures are born than can survive to reproduce (since all creatures require certain limited resources). So general those individuals best suited for survival and reproduction will survive and reproduce - and they are the ones who pass along their 'winning' genes.